Modern semiconductor device fabrication processes frequently use selective etching conditions that will etch one material more rapidly than another. The material that is etched more rapidly can be referred to as a sacrificial material, and the material that is etched less rapidly can be referred to as a protective or etch stop material. Selective etching can be use in processes in which it is desired to protect a portion of a semiconductor wafer from etching conditions while etching through another portion of the wafer. Exemplary selective etching conditions are dry etch conditions that are selective for etching silicon oxide relative to silicon nitride, including those described in U.S. Pat. No. 5,286,344, the disclosure of which is incorporated by reference herein in its entirety.
Many prior art selective etching methods generally have poor selectivities, such that there is a constant risk that the protective material will be etched away entirely during the etching of the sacrificial material. The selectivity for a given selective etching method is highly dependent on numerous factors, such as the composition of the film and the etching conditions. It would be desirable to develop alternative methods of selective etching having better selectivities under comparable conditions.
A possible mechanism by which selectivity can occur is through selective polymer formation on the protective material during etching of it and the sacrificial material. For instance, etching of silicon oxide and silicon nitride under conditions such as those described in U.S. Pat. No. 5,286,344 can create a carbonaceous polymer on the silicon nitride which protects the silicon nitride during etching of the silicon oxide. The carbon contained in the carbonaceous polymer can originate from, for example, etchant materials, such as gas, liquid or plasma materials, including, for example, CH.sub.2 F.sub.2 and CHF.sub.3. When silicon oxide, such as borophosphosilicate glass (also referred to throughout the specification as BPSG), is selectively etched relative to silicon nitride, the carbon will frequently originate at least in part from etching of the BPSG. Thus, less selectivity is obtained when less BPSG is etched relative to an amount of silicon nitride exposed to the etching conditions. Accordingly, thin layers of BPSG can be more difficult to etch than thick layers. Many selective etching methods are not effective for selectively etching BPSG relative to silicon nitride when the BPSG layers have thicknesses of less than or equal to about 1.3 microns.
An exemplary application of selective etching is a dynamic random access memory (DRAM) forming process. Referring to FIG. 1, a DRAM construction is illustrated with respect to semiconductive wafer fragment 10. Semniconductive wafer fragment 10 comprises a substrate 12, which can be, for example, a monocrystalline wafer lightly doped with a p-type background dopant. The term "semiconductive wafer fragment" refers to any construction comprising semiconductive materials, including, for example, bulk semiconductive materials, such as semiconductive wafers (either alone or in assemblies comprising other materials thereon) and semiconductive material layers (either alone or in assemblies comprising other materials). The term "substrate" refers to any supporting structure, including, for example, the semiconductive wafer fragments described above.
Field oxide regions 15 overlay the substrate 12, and node locations 14, 16 are between the field oxide regions 15. The node locations 14, 16 contain diffusion regions conductively doped with a conductivity-enhancing dopant.
Wordlines 20 overlay the substrate 12. The wordlines 20 comprise a gate oxide layer 24 and a conductive layer 26. The gate oxide layer 24 and conductive layer 26 may also be referred to as a conductive gate. The gate oxide layer 24 can comprise, for example, silicon dioxide. The conductive layer 26 can comprise, for example, conductively doped polysilicon capped with a metal silicide, such as, for example, tungsten silicide or titanium silicide. The wordlines 20 have opposing sidewall edges and sidewall spacers 28 (referred to herein as a "spacer" or "spacers") that extend along the sidewall edges. The spacers 28 can comprise, for example, silicon oxide or silicon nitride. An etch stop layer 32 extends over the wordlines 20. The etch stop layer 32 can comprise, for example, silicon nitride. Although not shown, an insulative layer can be placed between the etch stop layer 32 and the conductive layer 26. The insulative layer can comprise, for example, silicon oxide or silicon nitride.
An insulative layer 34 is provided over the substrate 12 and over the wordlines 20. The insulative layer 34 can comprise, for example, BPSG.
Capacitor constructions 36 extend through the insulative layer 34 to contact the node locations 14. The capacitor constructions 36 comprise a storage node or first electrode 40, a dielectric layer 42, and a second electrode 44. The storage node 40 and second electrode 44 can comprise, for example, conductively doped silicon such as a conductively doped polysilicon. The dielectric layer 42 can comprise, for example, silicon dioxide and/or silicon nitride. Although all the layers 40, 42, 44 are shown extending within openings in the insulative layer 34, other capacitor constructions can be used where some or none of the storage node, dielectric and second electrode layers extend within the opening.
A bit line contact 46 also extends through the insulative layer 34 and contacts a node location 16. The bit line contact 46 is in gated electrical connection with the capacitor construction 36 through the wordline 20. The bit line contact 46 can comprise, for example, tungsten, titanium and/or titanium nitride. Although not shown, a diffusion barrier layer, such as, for example, titanium nitride, can be formed between the bit line contact 46 and the diffusion region of the node location 16.
A second insulative layer 48 extends over the capacitor constructions 36 and electrically isolates the second electrode 44 from the bit line contact 46. The second insulative layer 48 can comprise, for example, silicon dioxide, BPSG or silicon nitride, and can be the same as or different from the first insulative layer 34.
A bit line 50 extends over the second insulative layer 48 and is in electrical connection with the bit line contact 46. Accordingly, the bit line contact 46 electrically connects the bit line 50 to the node location 16. The bit line 50 can comprise, for example, aluminum, copper or an alloy of aluminum and copper.
A method of forming the DRAM construction of FIG. 1 is described with reference to FIGS. 2 and 3. FIG. 2 illustrates a semiconductive wafer fragment 10 at a preliminary processing step. The etch stop layer 32 extends over the wordlines 20, and over the node locations 14, 16. The insulative layer 34 extends over the etch stop layer 32, and a patterned photoresist masking layer 60 is provided over the insulative layer 34. The patterned photoresist masking layer 60 defines an opening 62 which is to be extended to the node location 16 for ultimate formation of the bit line contact 46 therein.
Referring to FIG. 3, the opening 62 is extended to the etch stop layer 32. The etch used to extend the opening 62 is preferably selective for the material of the insulative layer 34 relative to that of the etch stop layer 32. For instance, if the insulative layer 34 comprises BPSG and the etch stop layer 32 comprises nitride, the etch can be a fluorocarbon, such as those described in U.S. Pat. No. 5,286,344.
After selectively etching to the etch stop layer 32, subsequent anisotropic etching of the etch stop layer 32 can occur to extend the opening 62 to the node location 16. Such extended opening can also be referred to as a self-aligned contact etch, referring to the opening being aligned with the sidewall edges of the wordlines 20. Preferred self-aligned contact etches are described in U.S. Pat. No. 5,286,344.
After the opening 62 is extended to the node location 16, the patterned photoresist masking layer 60 (shown in FIG. 2) can be removed, and subsequent processing used for forming the bit line contact 46 within the opening 62. Also, similar etching described above to form the opening 62 for the bit line contact can be used to form openings to the node locations 14 for formation of the capacitor constructions 36 therein. In the exemplified fabrication process, the opening 62 for the bit line contact is formed prior to forming openings for the capacitor constructions 36. However, other fabrication processes are known in the art wherein the openings for the capacitor constructions are formed either before, or simultaneously with, formation of the opening for the bit line contact.
FIG. 3 illustrates an idealized selective etch, where the etch stops substantially entirely upon reaching the etch stop layer 32. However, as discussed above, prior art etching processes are generally not much more selective for sacrificial materials (the material of insulative layer 34) than for protective materials (the material of etch stop layer 32). Accordingly, the selective etches do not generally stop substantially entirely upon reaching the etch stop layer 32, but rather continue at a slower rate upon reaching the etch stop layer 32.
FIG. 4 illustrates a prior art problem that can occur as a result of the continued etching of the etch stop layer 32. Specifically, the etch stop layer 32 can become thinned to an extent that the spacers 28 are exposed to the etching conditions. Such exposure can lead to etching through the spacers 28 to expose the conductive layer 26. In a bad scenario, the conductive layer 26 is shorted to the bit line contact 46 when the conductive material of the bit line contact 46 is formed within the opening 62. Also, the thinning of the etch stop layer 32 can lead to unpredictability during a subsequent etch of the etch stop layer 32 to expose the node location 16. It is then unknown how long to continue a subsequent etch. If the etch continues for too long, the etch can undesirably penetrate into the substrate 12, and possibly through the diffusion region at the node location 16.
As can be seen from the above discussion, there is a need in the art for improving the selectivity of the etching process in integrated circuit applications. The present invention is directed to these, as well as other, important ends.